In a Class AB amplifier the standing current through both transistors when there is no signal is referred to as idle current. The idle current decreases distortion in the cross-over region (the switch from one amplifier to the other). But the drawback is that it at the same time increases power consumption in the idle state. Another drawback is that the bias current applied to set the idle current has to be adjusted to the temperature.
Various attempts have been made to improve idle power consumption, without sacrificing high fidelity. For example, U.S. Pat. No. 4,077,013 discloses a system where the bias current is adjusted (down) when there is no input.
An attempt to overcome these problems is presented in the article: “Error correction and Non-Switching Power Amplifier Output Stages, by Hawksford, presented at the 102nd AES Convention, Mar. 22-25, 1997. With this approach, a high degree of linearity is achieved by current driving the output transistors and using a specific type of output transistors. However, as soon as the output leaves the class A condition the current in the branch not supplying current will turn off completely, i.e. running in pure class B operation.
Attempts have been made to actively monitor and control the bias current of each power transistor, for example U.S. Pat. No. 3,995,228 and U.S. Pat. No. 4,160,216 where the current is monitored by measuring the voltage across the emitter resistance, which causes problems especially when the idle current is small.
A solution is presented in U.S. Pat. No. 4,595,883. In this solution, the transistor which does not provide output will still remain active. This is achieved by a control loop that is misbalanced by the parasitic voltage dependent current of a transistor to keep an idle current running. However, this solution is not robust. The voltage dependent current which is referred to as the “Early voltage” is parasitic and normally a parameter with huge tolerances. The control circuit with local feedback of the audio signal further means that the audio signal has to pass several transistors before entering the output transistors. This feedback loop establishes a low impedance output at the emitters of the transistors. At this point there are two emitter resistors to measure the idle current. In order to have enough voltage across these transistors the emitter resistors have to be in the range of one Ohm, and the output stage would thus essentially have an output impedance of one Ohm. In a good amplifier, the output impedance must be at least 100 times lower. This can only be achieved by an external feedback control, in which case the one Ohm output resistor in combination with a reactive load may cause a phase shift and oscillation.
General Disclosure of the Invention
The object of the present invention is to provide an alternative solution to prior art solutions, providing a Class AB amplifier with controlled bias exhibiting low distortion and improved linearity.
According to the present invention, this and other objects are achieved by a method for controlling an idle current in an amplifier arrangement having a first and a second power amplifier, a positive voltage connected to a drain of the first power amplifier, a negative voltage connected to a drain of the second power amplifier, and an input signal connected to a gate of each power amplifiers, and wherein a source of the first power amplifier and a source of the second power amplifier are both connected to a load. The method comprises detecting a first drain current from the first power amplifier, detecting a second drain current from the second power amplifier, identifying an idle current as the smallest drain current of the first and second drain currents, comparing the idle current with a pre-set idle current set point, to provide an error signal, supplying the error signal to a bias control loop, to provide a feedback signal, driving a bias current proportional to the feedback signal through a resistor connected between the input signal and the gate of an inactive one of the first and second power amplifiers, the feedback signal thereby controlling a bias voltage on the gate to ensure that a drain current from the inactive power amplifier is equal to the pre-set idle current.
The objects are also obtained by an amplifier arrangement comprising a first power amplifier having a drain connected to a positive drive voltage and a gate connected to an input signal, a second power amplifier having a drain connected to a negative drive voltage and a gate connected to the input signal, wherein a source of the first power amplifier and a source of the second power amplifier are both connected to a load, a first current sensor detecting a first drain current from the first power amplifier, a second current sensor detecting a second drain current from the second power amplifier, processing circuitry adapted to identify an idle current as the smallest drain current of the first and second drain currents, a comparator for comparing the idle current with a pre-set idle current set point, to provide an error signal, a bias control loop providing a feedback signal based on the error signal, and means for driving a bias current dependent on the feedback signal through a resistor connected between the input signal and the gate of an inactive one of the first and second power amplifiers, so as to ensure that a drain current from the inactive power amplifier is equal to the pre-set idle current.
According to the invention a bias control loop will keep the idle current constant in the transistor with the lowest current (the inactive transistor). Thereby, the current running in the transistor which does not deliver current to the load will be fixed at a desired value. The value is fixed for all operation and signal conditions that might appear, and independent of the values of Vin, Vout, output current or a possible inductive load.
According to the invention, audio performance can be maintained with a lower idle current, thus leading to significant reductions in power consumption when idling. Alternatively, improved audio performance can be achieved without increasing the idle current.
Compared to a conventional class AB amplifier, the inactive transistor is still active, why an amplifier according to the present invention may be referred to as a “class ABA” amplifier.
It is noted that the expression “drain” implies that the transistors are MOS transistors. However, the present invention may also be implemented using bipolar transistors, in which case the collector current is detected, and the bias voltage is applied to the base.
The processing circuitry may be implemented by analogue circuitry or using a digital processor with suitable sampling.
According to a preferred embodiment, the bias current is applied to both transistors. This has a linearizing effect on the amplifier, and it can be shown that the non-linear component is reduced by 50%.